(1) Field of the Invention
The present invention relates to a process for producing hollow ceramic articles. More particularly, the invention relates to a process for producing hollow ceramic articles, such as ceramic port liners to be used for lining inner surfaces of exhaust ports for gasoline engines, diesel engines and the like, by drain or slip casting.
(2) Related Art Statement
It is already known that purifying performances of a catalyst are improved and turbo lag of a turbocharger is reduced by increasing temperatures of exhaust gases due to a heat insulating effect of a ceramic material lining the inner surface of an exhaust port in the form of a port liner. Such techniques are considered to be effective particularly for four valve type engines which are likely to reduce temperatures of exhaust gases. Such ceramic port liners may be shaped by a drain casting, in which a ceramic slurry is poured inside a water-absorbable mold made of gypsum or the like and an excess amount of the slurry is drained after a slurry material is deposited on the inner surface of the mold (For example, see "Ceramic Engineering Handbook" published by Gihoudo Co., Ltd. Dec. 25, 1966, pp. 1152-1159). However, in case of port liners, valve holes need to be bored at specific locations. Formerly, such valve holes are formed by mechanical working using a drill or the like after the shaping or firing. However, since a surface of the port liner to be worked is a curved surface, many shortcomings are pointed out that the working takes much time, that locating is difficult, that accurate working needs a great number of steps, and that cracks are likely to be formed due to brittleness of the ceramics even after the firing.
Further, although hollow ceramic articles having complicated configurations are generally shaped by drain casting which comprises the steps of pouring a slip inside a water-absorbable mold and draining the slip from the mold after a given time passes, the thickness of a deposited ceramic is adjusted by controlling a time period from a slurry pouring to a slurry drainage. Thus, when such hollow ceramic articles are to be mass-produced by using a number of molds, the deposition thickness is not kept constant due to variations in use history of the molds (gypsum molds), variations in water-absorbability due to changes in mold temperatures, and changes in viscosity and temperature of the slurry. Consequently, at the present time the thickness can be controlled only in the order of mm. Therefore, such a drain casting can be applied only to articles, such as novelties and sanitary wares, which do not require accurate thickness control.
Recently, ceramic port liners have been investigated to line inner surfaces of exhaust ports for automobile engines, and slip casting has attracted attention as a shaping process therefor. However, since the thickness of the ceramic port liner influences heat insulating properties and engine output, the thickness must strictly be controlled to not more than 0.1 mm. Such a requirement could not be satisfied by the conventional techniques at all.
For example, as shown in FIG. 9, a ceramic port liner has opening ends 14 and 15 on an engine side and an exhaust pipe side, respectively. Such a ceramic port liner is ordinarily shaped by drain casting as mentioned above. However, since aluminum titanate or the like usually employed as a material for the port liner has a great firing shrinkage factor, the shapes of the opening ends 14 and 15 are unfavorably likely to distort during firing. As shown in FIG. 9, since the four valve engine port liner has a particularly complicated configuration with two opening ends 14 on the engine side, these opening ends 14 are likely to distort. Further, firing shrinkage of the opening end 15 which contacts a setter during firing is restrained due to the self weight of the ceramic to cause distortion.
Further, when hollow ceramic articles are to be produced by drain casting, this technique has the shortcomings that if the grain size of a ceramic raw material in a slurry is great, or if the specific gravity of the raw material is large, or if the viscosity of the slurry is low, the raw material precipitates during deposition such that differences in deposited thickness occur between upper and lower portions of a shaped body. Thus, the raw material cannot be used after it is adjusted as a slurry suitable for casting by grinding and screening it.